Some of DC power supply apparatuses useable with arc-utilizing apparatuses are arranged to supply a DC voltage prepared by converting a commercial AC voltage. Commercial AC voltages in the worlds have various values. Such AC voltages may be classified into two categories, namely, a higher voltage group and a lower voltage group. The higher voltage group includes voltages of, for example, 380 V, 400 V, 410 V and 460 V, whereas the lower voltage group includes voltages of, for example, 200 V, 208 V, 230 V and 240 V. One of voltages of either higher voltage group or lower voltage group may be used in one country, but some countries may use, for example, voltages selected from each of the higher and lower voltage groups. In such countries or areas, it sometimes is difficult to determine which type of DC power supply apparatuses should be used, a DC power supply apparatus which is operable from a voltage of the higher voltage group or a DC power supply apparatus operable from a voltage of the lower voltage group.
Therefore, there is a need for a DC power supply apparatus which can operate from a voltage of either the higher or lower voltage group.
One example of such DC power supply apparatuses is disclosed in U.S. Pat. No. 5,272,313 issued to Karino et al. on Dec. 21, 1993. The DC power supply apparatus disclosed in this U.S. patent includes an input-side rectifier, which rectifies an input commercial AC voltage of, for example, 400 V or 200 V to a DC voltage. If the applied input AC voltage is a 400 V voltage, the rectified output voltage from the input-side rectifier is applied across a series combination of two capacitors. On the other hand, if the applied voltage is a 200 V voltage, the two capacitors are connected in parallel with each other, and the input-side rectifier output voltage is applied across the parallel combination of the two capacitors. The two capacitors smooth the applied rectifier output voltage, and the smoothed voltage across each of the capacitors is converted to a high frequency voltage in an inverter associated with that capacitor. The high frequency voltages from the two inverters are voltage-transformed by respective ones of two transformers, and the voltage-transformed voltages are rectified by an output-side rectifier.
The DC power supply apparatus disclosed in this U.S. patent can be modified so that it is usable in areas where a commercial AC voltage available is of either 230 V or 460 V. There are areas, however, in which a voltage of 575 V is used.
When the DC power supply apparatus operable from either 460 V or 230 V is used with a voltage of 575 V, the voltage resulting from the input AC voltage of 575 V rectified in the input-side rectifier should be smoothed by the series combination of the two capacitors and the voltage across each of the capacitors should be applied to the associated one of the inverters.
Assuming that the input AC voltage of 575 V to the DC power supply apparatus can vary by 12%, the peak value of the output voltage from the input-side rectifier is (575.times.1.12.times..sqroot.2).congruent.900 volts. Then, one-half of the peak value, i.e. about 450 V, is applied to each of semiconductor switching devices of each inverter. The maximum voltage rating of commercially available semiconductor switching devices is either 600 V or 1200 V. If a DC power supply apparatus with semiconductor switching devices for 600 V is used with an 575 V input commercial AC voltage, the withstand voltage of the semiconductor switching devices is too low, and, therefore, devices for 1200 V must be used. However, semiconductor switching devices for 1200 V are much expensive relative to devices for 600 V. In addition, inverters requires many semiconductor switching devices. Accordingly, the cost of such DC power supply apparatuses undesirably increases.
In order to overcome the above-described problem, it may be considered not to couple the 575 V input AC voltage to the serially connected capacitors across which the 460 V voltage would otherwise be applied, but to provide an additional circuit for converting the 575 V AC voltage to a DC voltage in addition to the above-described arrangement including the input-side rectifier, the capacitors, the inverters, the transformers and the output-side rectifier.
Assuming both the 575 V and 460 V AC voltages change by .+-.12%, the lowest possible value of the 575 V voltage is 506 (=575.times.0.88) volts, whereas the highest possible value of the 460 V voltage is 515 (=460.times.1.12) volts. That is, the variation ranges of the two voltages overlap. It is quite uneconomical to provide separate AC-to-DC converter circuits dedicated for the two voltages whose variation ranges overlap.
Japanese Patent No. 2,573,123 granted on Oct. 24, 1996, the grant being published on Jan. 22, 1997, discloses a DC power supply apparatus in which part of components are used in common for voltages whose variation ranges overlap.
According to the invention disclosed in this Japanese patent, however, regardless of whether the input AC voltage is of 575 V or 460 V, the DC power supply apparatus is arranged to supply a constant voltage to accessories, such as a fan for cooling the DC power supply apparatus and a wire feeding motor of an arc welder, but it does not switch major constituent circuits, e.g. inverters, of the DC power supply apparatus in accordance with the magnitude of the input AC voltage to supply the constant voltage.
Furthermore, no DC power supply apparatus has been proposed yet, which can be operable from any voltage selected from the lower voltage group including 200 V, 208 V, 230 V and 240 V voltages, a voltage in the higher voltage group including 380 V, 400 V, 410 V and 460V voltages, and a further higher voltage group including a 575 V voltage.
An object of the present invention is to provide a DC power supply apparatus for arc-utilizing apparatuses, which is operable from any one of voltages in the lower, higher and still higher voltage groups.